Oxo Fatty Acids
1H-NMR Spectroscopy of Fatty Acids and Their Derivatives
In a saturated fatty acid methyl ester, 10-oxostearic acid, the signals of the protons α to the oxo group were observed at 2.27-2.38 and 1.54-1.58 ppm, which included the signals of other protons such as those at C2 (Kuo et al., 1999). In methyl 3-oxohexadecanoate, the signal of the protons at C2 was shifted downfield to 3.45 as a singlet, while the other protons on a carbon α to the oxo group generated a triplet at 2.53 ppm (Lie Ken Jie and Lam, 1996). When the oxo group is close to the terminal group instead, a similar effect is observed for the terminal methyl protons.
In methyl 12-oxo-3(E)-tridecenoate, the terminal methyl protons caused a singlet at 2.13 ppm (Lie Ken Jie and Lam, 1996). The olefinic protons were observed as a multiplet at 5.53 ppm and the C2 protons as a doublet at 3.02 ppm.
Fatty compounds with vicinal carbonyl moieties (1,2-diones) gave triplets at about 2.70 ppm caused by the protons α to the carbonyl groups (Knothe, 2002). A dioxo compound with interrupting two methylene groups (1,4-diones), the characteristic signals were at 2.66 ppm (singlet, "inner", i.e. the interrupting CH2) and a triplet at 2.4-2.5 ppm (protons attached to the "outer" α carbons) (Lie Ken Jie and Lam, 1996).
Some data for monounsaturated oxo fatty acids (Porter and Wujek, 1987), showing that the presence of an allylic oxo group induces a strong downfield shift on double bonds, are:
|9-trans-11-oxooctadecenoic acid:||6.8 (dt, H9), 6.1 (dd, H10), 2.5 (t, H12)|
|10-trans-9-oxooctadecenoic acid:||6.8 (dt, H11), 6.1 (dd, H10), 2.5 (t, H8)|
|9-trans-8-oxooctadecenoic acid:||6.8 (dt, H10), 6.1 (dd, H9), 2.5 (t, H7)|
|8-trans-10-oxooctadecenoic acid:||6.8 (dt, H8), 6.1 (dd, H9), 2.5 (t, H11)|
Similarly, in methyl 12-oxo-10(E)-octadecenoate, a triplet was reported at 25 ppm for the protons at C-13 and the olefinic protons were observed at at 6.04 ppm (doublet of H-11) and 6.8 ppm (multiplet of H-10) (Lie Ken Jie and Syed-Rahmatullah, 1992). When the double bond is replaced by a triple bond, such as in methyl 8-oxo and 11-oxo-9-octadecynoate, the shift of the protons allylic to the triple bond is observed as a triplet at 2.36 ppm (Lie Ken Jie et al., 1997).
This effect is also visible in a compound such as 13-oxo-(9Z,11E)-octadecadienoic acid (Kuklev et al., 1997) in which the signals of the double bonds were observed at 5.9 ppm (ddt, H-9), 6.1 (ddd, H-10), 6.14 (dd, H-12), and 7.43 ppm (ddd, H-11) with the C8 allylic protons resonating at 2.28 as a mutiplet and the C14 protons giving a triplet at 2.55 ppm. Similar shifts were observed for 9-oxo-(10E,12Z)-octadecadienoic acid.
An oxo fatty ester with allenic unsaturation, methyl 12-oxo-9,10-octadecadienoate, displayed its characteristic shifts as a multiplet at 2.15-2.18 ppm for the C8 protons, a triplet at 2.56 ppm for the C13 protons, and two multiplets at 5.62 and 5.70 ppm for the lone protons at C9 and C11, respectively (Lie Ken Jie and Lau, 1999).
Unsaturated dioxo compounds of the type 2-ene-1,4-diones have been reported (Lie Ken Jie et al., 1997). In methyl 9,12-dioxo-10(Z)-octadecenoate, the olefinic protons were observed as a singlet at 6.32 ppm and the protons α to the enedione moiety resonated as triplet at 2.53 ppm. These shifts were downfield at 6.87 ppm and 2.64 ppm in the corresponding E isomer.
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